1. Field of the Invention.
The present invention relates to active power line conditioners which are utilized to regulate the quality of electrical energy delivered from an electrical energy source to an electrical load. More particularly, the invention relates to an improvement in such devices (APLC) which significantly increases the capability of rejecting common mode disturbances which may appear in the energy supply line.
2. Description of the Prior Art.
Electric supply networks are increasingly being exposed to the consequences of nonlinear loads, such as data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical apparatus and communication equipment. Such loads draw nonlinear pulse-like currents instead of the sinusoidal currents drawn by linear loads (i.e., resistors, inductors and capacitors). Such nonlinear currents flow through the source impedance of the electrical energy source, thus causing "normal mode" distortion of the AC line voltage.
This "normal mode" voltage distortion may produce a number of undesired effects. For example, sensitive loads connected to the network may experience operational difficulties. Additionally, the RM$ current supplied by the source increases due to the presence of harmonics in the nonlinear currents. These harmonics may significantly increase I.sup.2 R losses in interposing transformers.
Another problem which may have significant effects on many types of electrical equipment is the occurrence of temporary sags in the supply voltage. For example, electrical equipment in use today often incorporate a power supply input stage which converts the AC line voltage to DC voltage utilizing a full wave rectifier connected across one or more large filter capacitors. In normal operation, the filter capacitor recharges with each peak of the rectified line voltage. Only during this peak is the load actually drawing current from the electrical supply network. When the rectified line voltage is lower than the voltage level across the filter capacitors, rectifier diodes will prevent current from flowing. If, however, the AC line voltage does not maintain an adequate peak-to-peak level, these filter capacitors will not be able to maintain their required peak charge levels. As a result, operation of the associated electrical equipment may be affected.
Many of these problems can be mitigated through the use of power electronic devices known as active power line conditioners. Such devices typically comprise one or two switching inverters arranged in a series, parallel, or series-parallel configuration. The inverters are controlled (generally by pulse width modulation (PWM) techniques) to effect a flow of current between a DC energy storage element and the AC supply lines to which they are connected. Such devices are shown and described in U.S. Pat. Nos. 4,651,265 and 3,825,815, which are incorporated herein by reference.
In addition to normal mode disturbances, the electric supply network may be exposed to "common mode" disturbances frequently generated by lightning and ground fault switching tansients. Common mode disturbances, which appear between the power conductors and a ground conductor of the electric supply network, can exceed normal mode voltages by orders of magnitude. In addition to often causing damage to loads connected to the network, large common mode voltages can damage the APLC itself. It is thus desirable to reduce common mode disturbances to as great an extent as possible.
In the past, the elimination of common mode voltage disturbances has often been achieved by the installation of an isolation transformer. In order, however, to maintain the commercial viability of an APLC device, the manufacturing cost should be kept as low as possible. Thus, while an isolation transformer would likely work well for the purpose of eliminating common mode disturbances in an APLC device, they impose a significant weight and cost penalty which is undesirable.